Processes for the production of polymeric foams by reactive chemical routes are varied and well known. An example of such polymeric materials is flexible polyurethane foam which is produced in blocks typically 2 meters.times.2 meters.times.1 meter. These large blocks can be produced either continuously on conveyor type machines or discontinuously in molds.
Flexible polyurethane foam is formed by a reaction between a high molecular weight polyol and a diisocyanate. This reaction is highly exothermic reaching a peak, as depicted in a time/temperature curve, typically within about 5 to 30 minutes. Polyurethane foam therefore have to be transferred to an intermediate "cure area" promptly after initial cure where they are carefully stacked with air space around each block until they have cooled. A large area is required for this purpose and the blocks typically need to be stored for a minimum of 10 hours before they can be restacked or loaded for transporting to the customer. This process of intermediate storage, to ensure adequate cooling of the blocks, is inconvenient and costly in space requirements. Further, the intermediate storage area contains a large number of blocks of inflammable foam at high temperature, presenting a potential fire hazard. The building used for this intermediate storage needs to be specially constructed to meet fire regulations.
A further important factor is that certain of the foam forming reactions are reversible at high temperature, typically the allophanate reaction following the initial polyurethane bond formation. The blocks of foam in the intermediate storage area frequently, exhibit an internal temperature exceeding 140.degree. C. for several hours. If the ambient atmosphere in the intermediate area is not controlled, i.e. is of variable humidity, there is a potential for ingress of moisture into the block which will react with free isocyanate end groups and terminate them: EQU RNCO+H.sub.2 O.fwdarw.RNH.sub.2 +CO.sub.2
This reaction, removing the isocyanate required for the allophanate reaction, results in a reduced, uncontrolled level of cross linking in the foam and therefore a variable, reduced stiffness or compression hardness. In geographical locations where a high ambient humidity is common, it is known for foamers to increase the quantity of isocyanate in a given foam recipe deliberately, in spite of the cost penalty in doing so, so as to allow for the hardness loss that would otherwise be experienced.
It has been proposed in PCT/GB85/00605 (published No. WO 86/04017) to use a new approach of early cooling, and specifically a method of making blocks of polyurethane or other foam arising from exothermic reaction of foam-forming materials, wherein once the reaction has reached a desired stage of completion a gas of suitable composition and temperature is passed through the body of the block to carry away the heat of reaction. Other earlier proposals are those of Riccardi et al. U.S. Pat. No. 3,890,414 (published 1975) and Continental Gummi Werke German OLS 2,456,421 published 1976). The cooling gas as proposed will normally be air, and the approach is the reverse of the conventional approach of slow cooling and minimum exposure to air while cooling takes place.
The conventional approach has to be seen in the light of a long standing problem in the polyurethane foam industry, namely autoignition of foam blocks due to excessive heat caused by the exothermic polyurethane bond forming reaction. The problem occurs particularly with certain low density and high exotherm grades of foam, or foams containing additives which are included to render the foam resistant to small sources of ignition. Such foams can, after a period of two to three hours, and after they have started to cool, begin to increase in temperature again. This second exotherm is normally a self progressive type, eventually resulting in autoignition. Several factories have been burned down because of this phenomenon. One mechanism believed responsible for auto ignition is the drawing in of air from the atmosphere as the block cools. The oxygen enriched atmosphere within the block then causes exothermic oxidation of the polyurethane polymer with a resulting temperature rise. The presence of air drafts around the blocks has been shown to exacerbate the problem.
A newer approach is to deliberately draw cool air through an initially cooled polyurethane block. The composition of the air, at least as to moisture content, may be controlled, but such control alone has been found insufficient to achieve satisfactory results. Cool air drawn through the block removes heat, volatile gases, sublimates and excess water from the block.
Polyurethane foam commonly contains butylated hydroxy-toluene ("BHT" full name 2,6-ditertiarybutyl-4-methyl phenol), which is used as an antioxidant in the polyols that are reacted with isocyanates such as toluene diisocyanate ("TDI") to form the foam. BHT is a solid subliming at 70.degree. C. and therefore taken up in cooling air passed through blocks exhibiting an initial temperatures of 140.degree. C. or higher. The cooling air is desirably recycled for heat recovery, control of moisture content, and to prevent uncontrolled levels of residual isocyanate or auxiliary blowing agents such as chlorofluorocarbons ("CFCs") or other volatile compounds such as methylene chloride or 1,1,1-trichloroethane from reaching the atmosphere. For the purposes of the present specification, the terms "auxiliary blowing agent" and "auxiliary foaming agent" will be used interchangeably. Both of these terms refer to compounds added to a porous foam formulation in order to liberate a gas during subsequent chemical reaction. It is the liberation of gas that results in the foaming of the material.
In the past, air has been recycled through a heat exchanger. The heat exchangers have been rapidly blocked up with a solid deposit of BHT together with some polyurea formed from residual TDI and moisture. Other antioxidants and additives have caused similar problems.
U.S. patent application Ser. No. 07/531,958 (the "'958 application"), discloses another approach utilized for rapid cooling of porous materials. The process provides a partial solution for the problem with volatiles taken up from the hot material and later separating out and blocking heat exchangers. The '958 application discloses a process wherein the blocking problem is prevented by mixing heated gases coming from a first part of a cooling zone, and carrying the volatiles, with cold gases extraneous or coming via a heat exchanger from a second or subsequent part of the cooling zone, so that the volatiles separate out. The mixed gases which are passed through the porous material in the second or subsequent part of the cooling zone are substantially filtered within the porous material rather than in the heat exchanger.
The '958 application discloses a process and plant for cooling of porous materials. Specifically, the process concerns blocks of polyurethane or other open cell foamed plastics prepared from an exothermic reaction of foam forming materials. In the process, volatiles within the porous blocks are taken up by cooling gases and separate out therein on cooling of the gases below a separation (i.e. sublimation or condensation) temperature, characterized by:
i) effecting the cooling of the porous materials in two or more successive zones,
ii) mixing gases emerging from the first zone, carrying the volatiles, with gases at a lower temperature, particularly gases emerging from the second or subsequent zones and thereafter cooled by heat exchange, whereby the temperature of the mixed gases is brought below the separation temperature, and
iii) passing the mixed gases through the porous material to filter out the separated volatiles.
Thus, first gases are passed through the porous material in a first zone to cool the porous material and to remove volatiles therefrom in a first gas mixture which exits the porous material. The flow rate of the first gases through the porous material is controlled to provide a controlled, uniform cooling rate thereof. This first gas mixture is combined with second gases having a lower temperature to form a second gas mixture having a temperature which is sufficiently low to condense one or more of the volatiles.
Finally, the second gas mixture is passed through the porous material in a second zone to filter condensed or sublimed volatiles thereon and to further cool the porous material.
It is also desirable to remove particulate matter from the first gas mixture prior to mixing with the second gases. Also, particulate matter may optionally be removed from the second gases prior to mixing with the first gases. This removes certain materials from the gas streams and prevents later buildup on the heat exchangers which are utilized to cool the gas.
Even though the '958 application discloses an improved method for cooling polyurethane foam, several disadvantages still remain. The process of that application utilizes a heat exchanger in order to provide cool air for both reducing the temperature of a first porous block as well as to force the condensation of volatile material within a second porous block. Although the process utilizes the second porous block to effectively filter much of the BHT and TDI (TDI plus moisture forming polyurea) that had clogged heat exchangers and recuperators utilized in past apparatus, a portion of the BHT and TDI still passes unfiltered through the block. Therefore, over a prolonged period of time, clogging of the heat exchanger and recuperator will still occur.
Conventional foaming materials contain a substantial amount of solvent or auxiliary blowing agents such as chlorofluorocarbons which are released into cooling gas currents during the exothermic curing cycle. High concentrations of such compounds in the cooling gas necessitates recycling if emission of pollutants is to be avoided.
Water is a preferred foaming agent for forming polyurethane foam. Water reacts with an isocyanate group to form an intermediate carbamic acid which liberates carbon dioxide gas. However, as desirable as water is during the foaming phase of polyurethane form production, it may later deleteriously effect the foam by terminating free isocyanate groups as discussed above. Thus, water is normally used in combination with other blowing agents.
A uniform distribution of free isocyanate groups is required for producing foam with a predictable degree of cross-linking (and therefore stiffness and hardness). It would be desirable to utilize an increased amount of water in the polyurethane formulation so as to provide sufficient foaming without utilizing toxic auxiliary foaming agents. It would be of further advantage to simultaneously eliminate excess water during later curing of the foam which would otherwise interfere with polyurethane cross-linking.
In the past, a decreased amount of water has been utilized in polyurethane foam formulations in order to prevent the above-discussed interference with polyurethane cross-linking. In order to ensure adequate foaming in such formulations, auxiliary blowing agents such as the chlorofluorocarbons were utilized. As explained above, such agents present in foam formulations in large quantities pose a toxic threat and require recycling.
What is needed is a method of rapidly cooling polyurethane foam wherein the heat liberated during exothermic reaction is controlled so as to prevent slow oxidation or outright ignition of the foam. At the same time, a method for fabricating polyurethane foam is needed wherein the polyurethane foam formulation includes a greater quantity of water as a foaming agent without causing interference with cross-linking of the polyurethane, thereby reducing or eliminating the need for utilizing toxic auxiliary foaming agents.